The present invention relates generally to lighting devices, and more particularly to an organic light emitting diode.
Organic electroluminescent devices, such as organic light emitting diodes (OLEDs), are currently used for display applications and are planned for use in general lighting applications. An OLED device includes one or more organic light emitting layers disposed between two electrodes, e.g., a cathode and a light transmissive anode, formed on a light transmissive substrate. The organic light emitting layer emits light upon application of a voltage across the anode and cathode. Upon the application of a voltage from a voltage source, electrons are directly injected into the organic layer from the cathode, and holes are directly injected into the organic layer from the anode. The electrons and the holes travel through the organic layer until they recombine to form excited molecules or excitons. The excited molecules or excitons emit light when they decay.
However, the external quantum efficiency of OLEDs, which is defined as a ratio of the photons emitted by the device to the number of injected electrons is lower than desired. There have been prior attempts to improve the external quantum efficiency of OLEDs by increasing the number of light beams that strike the substrate/air interface at an angle less than the critical angle.
For example Lai et al. (CLEO Conference Proceedings, Pacific Rim 99, WL6, pages 246-47 (1999)) suggests texturing the bottom light emitting surface of a glass substrate (i.e., the surface distal from the OLED device). The textured surface enables more light rays from the organic light emitting layer to strike the substrate/air interface at an angle smaller than the critical angle, thus allowing more light rays to escape from the substrate.
Furthermore, Gu et al. (22 Optics Letters 6, 396 (1997)) proposed forming deep grooves in the top surface of a glass substrate (i.e., the substrate surface adjacent to the OLED device). The glass/air interface of the grooves reflects the light emitted by the organic layer toward the bottom light emitting surface of the substrate. The reflected light increases the number of light beams that strike the bottom substrate surface/air interface at an angle less than the critical angle. Therefore, the total amount of transmitted light in increased. However, Gu et al. note that when the grooves were filled with a refractive index matching fluid (i.e., a fluid having the same refractive index as the substrate), the improvement in the external quantum efficiency is destroyed because the reflection of light at the groove/air interface is eliminated. Thus, the external quantum efficiency of an OLED having substrate grooves filled with an index matching liquid is the same as that of an OLED with a flat substrate. Furthermore, the device of Gu et al. has a low fill factor, which is defined as a ratio of the active device area to total device area. The device of Gu et al. has a maximum potential fill factor of 0.3 and an actual fill factor of the device illustrated in the Figures of this reference of 0.0085. While this fill factor is acceptable for display devices, a larger fill factor is desired for general lighting applications.
However, despite the efforts, the external quantum efficiency of OLEDs, such as those of Lai et al. and Gu et al., is still lower than desirable. The present invention is directed to overcoming or at least reducing the problem set forth above.
In accordance with one aspect of the present invention, there is provided an organic electroluminescent light emitting device, comprising a first electrode, a second electrode, at least one organic light emitting layer, and an output coupler which reduces a Fresnel loss.
In accordance with another aspect of the present invention, there is provided an organic electroluminescent light emitting device, comprising a first electrode, at least one organic light emitting layer over the first electrode, a second transparent electrode over the at least one organic light emitting layer, and a shaped transparent material which has corrugated or dimpled light emitting surface and which contains nanoparticles having a size of less than 100 nm over the second transparent electrode.
In accordance with another aspect of the present invention, there is provided an organic electroluminescent light emitting device, comprising a first electrode, at least one organic light emitting layer over the first electrode, a second transparent electrode over the at least one organic light emitting layer, and a matrix material containing light scattering particles over the second electrode.
In accordance with another aspect of the present invention, there is provided a method of making an organic electroluminescent light emitting device, comprising placing a first material into a mold cavity, solidifying the first material to form a shaped transparent material having corrugated or dimpled first light emitting surface, and attaching the shaped transparent material to the an organic electroluminescent light emitting device.
In accordance with another aspect of the present invention, there is provided an organic electroluminescent light emitting device, comprising a first electrode, at least one organic light emitting layer over the first electrode, a second transparent electrode over the at least one organic light emitting layer, and a shaped transparent material whose index of refraction is selected to match that of an adjacent layer of the light emitting device.